The Basics and the Application of Solar Desalination
Desalination is the process of removing dissolved salts and minerals from salty water. It is an important process for freshwater generation – especially given the fact that freshwater stocks around the world are coming under increasing strain.
Interestingly, desalinated water must be re-mineralized before it is fit for human consumption, as the water initially produced by desalination is distilled. Drinking such water would deplete human beings of minerals in their bodies.
Desalination is carried out through two main processes:
A) Thermal Processes involve the use of heat energy to boil water and thereby separate the water from salt or any other impurities (such as sodium, calcium, chloride and carbonate) in it. It is further classified into:
i) Multi-Stage Flash (MSF) Desalination: Impure water is passed through an evaporator consisting of several chambers maintained at progressively decreasing pressures. Water turns to vapour in each chamber, which is condensed and collected as fresh, distilled water.
ii) Multi-Effect Desalination: It is similar to the MSF process, except that it utilizes successive evaporation vessels.
iii) Vapour Compression Desalination: In this process evaporation of water is achieved by the application of heat from compressed water vapour. This process is used to produce freshwater for small/medium scale applications such as petroleum drilling sites, resorts etc.
iv) Solar Desalination: Uses the energy of the sun to boil water, which is then condensed into freshwater and collected.
B) Membrane Technologies involve the use of a membrane/filter to remove salt and other impurities.
i) Reverse Osmosis: Water is forced through a small pored filter (a complex, precisely manufactured polymer-based membrane) that sifts out impurities.
ii) Electrodialysis: A strong electrical current is run through the water. The process employs membranes such that only positively charged ions (cations) or negatively charged ions (anions) are allowed to permeate through.
iii) Membrane Distillation: Water is heated to increase its vapour pressure, which then differs from the partial pressures on both sides of a filtration membrane. Water vapour passes through the pores of a hydroponic membrane, where on the other side it is condensed on a cooler surface to generate fresh water.
Solar Desalination: a Promising Future Technology
Solar desalination (using the energy of the sun, through photovoltaic electricity or solar thermal energy to power a desalination unit) is a versatile new technology that can be paired with almost all existing desalination technologies to achieve cost-effective fresh water production.
An example of the pairing is that of the WaterFX solar thermal desalination plant in Central Valley, California. The plant converts contaminated and salty irrigation runoff into distilled water. It is powered by an array of parabolic trough collectors (PTCs).
It currently operates at a rate of 14,000 gallons (53,000 liters) per day, but is capable of being scaled up to an output of 2 million gallons (757,083 liters) per day.
What is most impressive is that it desalinates water at a quarter of the cost ($450 per acre foot*) of conventional technologies such as membrane distillation or reverse osmosis (nearly $2,000 per acre foot).
*An acre foot is a volume measure defined as an area of one acre, filled with water 1 foot deep.
It’s important to understand here that the economics of each particular technology varies according to the region of application.
For example, in developing countries such as India, with unreliable or little access to grid electricity, solar powered electrodialysis works out to be much more economical than reverse osmosis. However the reverse is true at facilities that are connected to reliable grid power.
In any case, solar desalination will be important in the near future. The cost of conventionally generated electricity continues to rise, and with the world population estimated to hit 9 billion by 2050, a lot more fresh water will be required. It will be interesting to see, if solar desalination figures in discussions at the upcoming COP21 in Paris.
To read more about solar desalination, click here.